1. Field of the Disclosure
The present invention relates to a glass having excellent resistance against surface damages as well as a method for the production thereof.
2. Description of Related Art
For the production of flat glass, the floating method is normally used due to its economic efficiency and the high quality of the surfaces of the flat glasses produced. In the production of flat glasses according to the floating method, the shaping is conducted by casting onto a liquid metal, normally tin, in a reducing atmosphere of forming gas. Floating facilities normally consist of a melting tank in which the glass is molten and refined, a transfer line which guarantees the transfer of the glass melt from the oxidizing atmosphere of the melting tank to the reducing atmosphere of the subsequent floating section, and the real floating section. In this floating section the shaping of the glass takes place. The shaping is effected by flat-flowing on the tin bath and by so called top rollers which exert force to the surface of the glass. During the transport on the tin bath, the glass cools down and at the end of the floating section it is lifted off and transferred into an annealing leer for thermal stress relief.
Starting with this transfer from the floating section into the annealing leer, the surface of the floating glass, in particular the floating lower contact side is subject to mechanical stress. Particularly, the first lift-out rollers after the lift-off operation are critical. In this zone the surface of the glass is particularly sensitive for mechanical stress by contact with the lift-out rollers due to the high temperatures which are near the transformation temperature Tg of the glass. Thereby, point scratches may be formed. Through the higher adhesive tendency of the hot surface of the glass at the contact areas, chips and/or digs may be formed. Besides the visual impairment, the surface damages also result in lowering of strength and in disadvantages in further processing steps, for example in a coating process.
Prior art provides a preventive measure which is particularly suitable for alkali-containing glasses. Thereby, sulfur trioxide gassing is conducted. Such a treatment of the lower side with sulfur trioxide vapors in the annealing leer is described in DE 1 596 514 C3. During the gassing procedure with sulfur trioxide, sodium oxide and sulfur dioxide form sodium sulfate. Subsequently, sodium sulfate can be removed from the glass by washing. During the sulfur trioxide gassing procedure, alkali sulfate containing precipitations on the surface of glass are formed through a reaction of alkali constituents of the floated glass. These salts provide a particular protection against mechanical stress during the lift-off operation and at the subsequent lift-out rollers through their low hardness. This sulfur trioxide gassing procedure has disadvantages due to different reasons. For an effective gassing procedure with sulfur trioxide, an oxidizing atmosphere is necessary. Due to this reason, often the glass is only subject of the gassing procedure with sulfur trioxide at the point of the transfer into the annealing leer. The zone of the lift-out rollers which is still often under reducing atmosphere remains unprotected. The protection mechanism has lower efficacy in the case of alkali free or low-alkali glasses, such as e.g. display glasses. Also particular preventive measures by technical installations have to be taken to limit the content of sulfur trioxide in the environment. These protective measures are technically intricate and during the post processing purification from adherent salts is usually necessary as an additional step.